Method of producing artificial leather and artificial leather

ABSTRACT

The present invention relates to a method of producing artificial leather, comprising steps of: (a) providing a substrate; (b) providing a foamed polyurethane dispersion, wherein the polyurethane dispersion has a solid content of about 40% to about 60% and a void content of about 0.01% to about 80%; (c) coating the substrate with a layer of the foamed polyurethane dispersion; and (d) drying the substrate and the layer of the foamed polyurethane dispersion for obtaining the artificial leather. Artificial leather is also provided in the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing artificialleather and artificial leather.

2. Description of the Related Art

Artificial leather, also known as synthetic leather, provides aneconomic and multi-functional way to replace natural leather. Artificialleather comprising a polyurethane formed on a substrate such as a wovenfabric or non-woven fabric has been used widely for many years. Suchartificial leather is required to be qualified in stain resistance,waterproofness, abrasion resistance and non-tackiness, and depending onthe application, is also required to be low in the coefficient ofsurface friction.

Conventionally, polyurethane is formed on the substrate by wet coating.Wet coating is performed by coating a polyurethane solution onto asubstrate 11 to be coated, and then drying and curing the polyurethaneto form a membrane or layer 12 on the substrate (referring to FIG. 1). Aconventional solvent for dissolving polyurethane to form a polyurethanesolution is dimethylformamide (DMF). When curing the polyurethanesolution applied on the substrate, water is usually added for replacingdimethylformamide, and polyurethane is thus cured. In conventionalprocesses, the use of dimethylformamide results in serious environmentalproblems, such as air pollution and water pollution. Furthermore,dimethylformamide is proven in connection with several healthdysfunctions. The quality of the polyurethane layer obtained by wetcoating is not satisfied either. The thickness of the coating islimited. Holes formed inside the layer of polyurethane on the substrateare about in a drop shape, and its size is quite large and with a lengthof about 250 to about 350 μm and a width of about 150 to about 320 μm.Hole distribution is also uneven; wherein larger holes are present in apart near the surface of the substrate. Such product fails to meet therequirement of mechanical strength (such as peeling, abrasion andflexibility). The bonding strength of the conventional productmanufactured by wet coating is not satisfied. Furthermore, the conditionof wet coating is limited and difficult to be controlled. In addition,folding lines easily appear on the polyurethane layer of theconventional artificial leather, and the life of usage is shortened.

U.S. Pat. No. 5,460,873 discloses a novel cover material for anintegrally foamed article comprising a permeable fabric and a thin layerof latex foam integrally bonded to the back surface of the fabric. Thelatex foam has numerous fine open cells which are formed in an internalpart thereof and further finer or denser open cells which are formed ina region adjacent to its outer surface. On the outer surface of latexfoam layer is formed a superficial skin. In a method for manufacturingthe novel cover material of this patent, the latex material issufficiently stirred to produce a latex foam with a density as low aspossible (with a density in the range of from 20,000 cells per cm² to70,000 cells per cm²). After drying and curing, the entire fabric issubjected to a crushing operation in which the fine open cells of thelatex foam are crushed and conveniently reduced to finer or denser opencells so as to increase permeability of the latex foam. The latex foamis designed to be a gas permeable material, and the physical andchemical properties of the latex foam are quite different from those ofartificial leather. For example, because the latex foam is covered withthe superficial skin, the appearance and flatness of the latex foam arenot strictly required. Furthermore, the cell size formed in the latexfoam layer is still uneven.

Given the above, a method of producing high-quality artificial leatherand the artificial leather thus produced are needed in this field.Especially, no toxic substance is used in the method of producingartificial leather or present in the artificial leather obtainedtherefrom.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method of producingartificial leather, comprising steps of:

-   -   (a) providing a substrate;    -   (b) providing a foamed polyurethane dispersion, wherein the        polyurethane dispersion has a solid content of about 40% to        about 60% and a void content of about 0.01% to about 80%;    -   (c) coating the substrate with a layer of the foamed        polyurethane dispersion; and    -   (d) drying the substrate and the layer of the foamed        polyurethane dispersion for obtaining the artificial leather.

Another object of the present invention is to provide artificial leathercomprising:

-   -   a substrate having a surface; and    -   a layer of foamed polyurethane formed on the surface of the        substrate, comprising a plurality of holes distributed evenly        therein, wherein the cross section of the holes is substantially        in an ellipse shape; preferably, the holes in an ellipse shape        has a diameter of about 20 μm to about 150 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the cross-section view under a scanning electronmicroscope of a conventional artificial leather having the holes with alength of about 250 to about 350 μm and a width of about 150 to about320 μm.

FIG. 2 illustrates the cross-section view under a scanning electronmicroscope of the artificial leather according to the present inventionhaving the holes with a diameter of about 20 μm to about 150 μm.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of producing artificial leather,comprising steps of:

-   -   (a) providing a substrate;    -   (b) providing a foamed polyurethane dispersion, wherein the        polyurethane dispersion has a solid content of about 40% to        about 60% and a void content of about 0.01% to about 80%;    -   (c) coating the substrate with a layer of the foamed        polyurethane dispersion; and    -   (d) drying the substrate and the layer of the foamed        polyurethane dispersion for obtaining the artificial leather.

As used herein, the term “substrate” refers to a base material allowinga polyurethane layer to be formed on a surface thereof. The substrateaccording to the invention is a non-woven fabric or a woven fabric. Asused herein, the term “fabric” refers to a single fabric or compositefabric, preferably, composite fabric. Artisans skilled in this field canchoose suitable types of fabric according to the disclosure of thespecification.

Preferably, the non-woven fabric used as a substrate is an ultrafinenon-woven fabric, a needle-bonded fabric or a water-jet entanglingfabric. The fineness of the fiber is preferably 0.01 denier to 10denier. More preferably, the non-woven fabric is a sea-island typecomposite fabric comprising a sea component and an island component. Thesea-island type composite fabric used in the present invention isproduced by conjugate-spinning or blend-spinning two polymers in asea-island manner. In one preferred embodiment of the invention, thesea-island type composite fabric is treated by weight reducing to reducethe amount of the sea component. The manner of weight reducing the seacomponent is well known to artisans skilled in this field, such as usingsodium hydroxide or toluene.

Preferably, the non-woven fabric is made of at least one materialselected from the group consisting of nylon, poly(ethyleneterephthalate) (PET), polyolefin, polymethacrylate, polymethylmethacrylate, acrylic resin, and the mixture thereof.

In another aspect, the woven fabric is a stretch fabric, preferably, aknitted stretch fabric. In one preferred embodiment of the invention,the stretch fabric comprises stretch yarn or an elastic component.

In order to achieve better mechanical and chemical properties, thesubstrate is preferably pretreated by impregnating or coating thesubstrate with a resin solution such as an elastomer solution comprisingelastomer resin, and additives such as dyes or color fixing agents.Preferably, the resin solution comprises acrylate resin or polyurethaneresin. In one preferred embodiment of the invention, the acrylate resincomprises polymethacrylate, polymethyl methacrylate or acrylic resin. Inanother preferred embodiment of the invention, the polyurethane resincomprises organic polyurethane resin or polyurethane dispersion resin.The manner of impregnating or coating is well known to artisans skilledin this field, which is incorporated hereinwith for reference.

In one preferred embodiment of the invention, a washing step and adrying step are performed after the impregnating step in the resinsolution. The manner of washing and drying are well known to artisansskilled in this field. In one embodiment of the invention, water is usedin washing and extrusion wheels are optionally used. The conditions forwashing are well known to artisans skilled in this field. Preferably,the substrate is washed in water and then subjected to the extrusionwheels several times. The conditions for drying are well known toartisans skilled in this field. In one embodiment of the invention, thedrying is air-drying. Preferably, the substrate is dried at 90° C. to170° C., more preferably, 120° C. to 160° C., most preferably, 130° C.to 150° C.

As used herein, the term “polyurethane” collectively means polyurethane,polyurea, polyurethane-polyurea, and the mixture thereof.

The foamed polyurethane dispersion according to the invention is formedby a mechanical or chemical manner. In one preferred embodiment of theinvention, the foamed polyurethane dispersion is formed by mechanicallystirring a polyurethane dispersion. Through mechanically stirring, airis mixed into the dispersion homogenously. The void content of thefoamed polyurethane dispersion according to the invention is easilycontrolled by adjustably conducting mechanical stirring of thepolyurethane dispersion. On the other hand, the foamed polyurethanedispersion can also be formed by using a foaming agent. The content orspecies of the foaming agent is well known to artisans skilled in thisfield. Preferably, the foaming agent is selected from the groupconsisting of sodium bicarbonate, ammonium carbonate, ammoniumbicarbonate, sodium borohydride, sodium nitrite, polyvinyl chloride,ethylene vinyl acetate, N-nitroso compounds, azo compounds,sulfonhydrazide, and the mixture thereof.

Solid content of the foamed polyurethane dispersion is unexpectedlyfound to play an important role in mechanical and chemical properties ofthe artificial leather. The foamed polyurethane dispersion according tothe invention is characterized by having a solid content of about 40% toabout 60%. Conventionally, the solid content of polyurethane dissolvedin an organic solvent ranges from 10% to 30%. The solid content of thefoamed polyurethane dispersion in the present invention is higher thanthat of polyurethane solution conventionally used. Preferably, thepolyurethane dispersion according to the invention has a solid contentof about 50% to about 60%, more preferably, about 55%.

In another aspect, void content of the foamed polyurethane dispersion isalso unexpectedly found to play an important role in mechanical andchemical properties of the artificial leather, especially in hardnessand mechanical properties. The foamed polyurethane dispersion accordingto the invention is characterized by having a void content of about0.01% to about 80%, preferably, about 2% to about 65%.

In one preferred embodiment of the invention, the foamed polyurethanedispersion further comprises an additive for mediating its specificfunctions. Examples of the additive are a dye, a thickener, anantioxidant, a flame-resisting agent, an anti-yellowing agent, anantibiotic, or the mixture thereof. The content or species of theadditive is well known to artisans skilled in this field, which isincorporated hereinwith for reference.

The manner of coating the substrate with the foamed polyurethanedispersion is well known to artisans skilled in this field. Examples ofcoating include direct applying, such as knife-coating and roll-coating.After coating, a layer of the foamed polyurethane dispersion is formedon the surface of the substrate.

The manner of drying the substrate and the layer of the foamedpolyurethane dispersion is well known in this field. In one preferredembodiment of the invention, the substrate and the layer of the foamedpolyurethane dispersion are dried by heating. The drying step also leadsthe foamed polyurethane dispersion to cure and form a layer of foamedpolyurethane on the surface of the substrate.

A reprocess performed on a surface of the foamed polyurethane layer ispreferably desired. The method according to the invention furthercomprises:(e) post-treating the artificial leather by the step ofadhering a layer, stamping, grinding, printing, or spraying on a surfaceof the artificial leather. Such reprocess achieves different appearanceor properties of the surface of the artificial leather. The manner ofstamping, grinding, printing, or spraying is well known to in thisfield, which is incorporated hereinwith for reference.

The method according to the invention avoids the defects of conventionalmethods of using a toxic solvent, dimethylformamide. The foamedpolyurethane dispersion used in the present invention is an aqueoussolution and has no harm to the environment or human.

The present invention also provides artificial leather producedaccording to the method as mentioned above. The artificial leathercomprises:

-   -   a substrate having a surface; and    -   a layer of foamed polyurethane formed on the surface of the        substrate, comprising a plurality of holes distributed evenly        therein, wherein the cross section of the holes is substantially        in an ellipse shape; preferably, the holes in an ellipse shape        has a diameter of about 20 μm to about 150 μm.

The cross-section view under a scanning electron microscope of one typeof the artificial leather according to the present invention is shown inFIG. 2; wherein 21 represents the substrate, 22 represents the foamedpolyurethane layer, and 23 represents the adhering layer as describedbelow.

In another aspect, the artificial leather with the foamed polyurethanelayer according to the invention comprises a layer attached on a surfaceof the foamed polyurethane layer on the side opposite to the substrate.The layer is an adhering layer, a printing layer, a spraying layer,etc., depending on the desired application and reprocess involved in theproduction method. In still another aspect of the invention, the foamedpolyurethane layer has a surface having texture thereon. The texture ispreferably produced by stamping or grinding.

The artificial leather according to the invention dramatically improvesthe mechanical and chemical properties thereof as compared toconventional artificial leather. The hole shape and size according tothe invention are also improved. The conventional artificial leather hasthe drop-shaped holes with a length of about 250 to about 350 μm and awidth of about 150 to about 320 μm; on the other hand, the artificialleather according to the invention has ellipse holes has a diameter ofabout 20 μm to about 150 μm. The ellipse shape and smaller size of theholes of the foamed polyurethane layer of the artificial leatheraccording to the invention avoid the defects of folding lines ofconventional artificial leather. Furthermore, the mechanical strength(such as peeling, abrasion and flexibility) is improved. It is notedthat the peeling strength of the artificial leather according to theinvention is above 2.5 kg/cm. No folding lines appear on the foamedpolyurethane layer of the artificial leather according to the inventionand the surface of the artificial leather is fine and flat. Theartificial leather according to the invention has excellent propertiesand is qualified to replace natural leather in the application ofmanufacturing shoes, furniture, cloth, and the like.

The following examples are given for the purpose of illustration onlyand are not intended to limit the scope of the present invention.

EXAMPLE 1

Substrate. Composite fibers of nylon and polyethylene terephthalate(PET) with a fineness of 3 denier were needle-bonded to form a non-wovenfabric substrate. The thickness of the substrate was 2.0 mm and theweight of area unit was 900 g/m².

Substrate Impregnating. An elastomer resin solution comprising 7 wt %polyurethane, 1 wt % dye and 92 wt % dimethylformamide was prepared andthe substrate was impregnated, in the elastomer solution.

Substrate Curing. The substrate, after impregnating, was put in water tocure the resin impregnated in the fibers.

Substrate Washing. The residues and the excess water were removed byextrusion wheels. The substrate was then washed in water and thensubjected to the extrusion wheels for several times.

Substrate Drying. The substrate, after washing, was then dried.

Substrate Reducing. The substrate was then treated with 5 wt % sodiumhydroxide for hydrolyzing the long chain of PET. After removing theexcess sodium hydroxide, washing and drying the substrate, the finenessof the fabric was reduced to 0.07 denier.

Coating. The 2.0 mm semi-product was coated with a foamed polyurethanedispersion, formed by mechanically stirring, having a void content of10%, a foaming density of 0.9 g/cm³ and a solid content of 52%, 0.3 wt %dye, and 5.4 wt % thickener.

Drying. The substrate with the foamed polyurethane dispersion was thendried for removing excess water and forming a layer of the foamedpolyurethane on the surface of the substrate.

Reprocessing. A releasing paper with natural leather texture was coatedwith a polyurethane layer and then further with polyurethane adhesive toform an adhering layer. The surface of the foamed polyurethane layer onthe side opposite to the substrate was then adhered with the adheringlayer. After curing, the release paper was released to obtain theartificial leather.

EXAMPLE 2

Substrate. Composite fibers of 60% nylon and 40% polyethyleneterephthalate (PET) were needle-bonded to form a non-woven fabricsubstrate. The thickness of the substrate was 1.0 mm and the weight ofarea unit was 280 g/m².

Substrate Impregnating. An elastomer resin solution comprising 10 wt %polyurethane dispersion, 3 wt % dye, 2 wt % thickener, and 85 wt % waterwas prepared and the substrate was impregnated in the elastomer resin.

Substrate Drying. The substrate was then dried to form a semi-productwith a thickness of 1.0 mm.

Coating. The semi-product was coated with a foamed polyurethanedispersion having a void content of 15% and a solid content of 52%, 0.3wt % dye, and 5.4 wt % thickener.

Drying. The substrate with the foamed polyurethane dispersion was thendried for removing excess water and forming a layer of the foamedpolyurethane on the surface of the substrate.

Reprocessing. A releasing paper with natural leather texture was coatedwith a polyurethane layer and then further with polyurethane adhesive toform an adhering layer. The surface of the foamed polyurethane layer onthe side opposite to the substrate was then adhered with the adheringlayer. After curing, the release paper was released to obtain theartificial leather.

EXAMPLE 3

Substrate. Single fibers polyethylene terephthalate (PET) wereneedle-bonded to form a non-woven fabric substrate. The thickness of thesubstrate was 1.0 mm and the weight of area unit was 330 g/m².

Coating. The semi-product was coated with a foamed polyurethanedispersion having a void content of 15% and a solid content of 52%, 0.3wt % dye, and 5.4 wt % thickener.

Drying. The substrate with the foamed polyurethane dispersion was thendried for removing excess water and forming a layer of the foamedpolyurethane on the surface of the substrate.

Reprocessing. A releasing paper with natural leather texture was coatedwith a polyurethane layer and then further with polyurethane adhesive toform an adhering layer. The surface of the foamed polyurethane layer onthe side opposite to the substrate was then adhered with the adheringlayer. After curing, the release paper was released to obtain theartificial leather.

Assay

The appearance, peeling, abrasion and hydrolysis resistance of theartificial leather obtained in Examples 1 to 3 were assayed. The methodsand apparatuses for assaying are listed below:

Abrasion: ASTM D3886, CSI Cs-022C S/N-846

Peeling: ASTM D751, Instron 4465

Hydrolysis resistance: ASTM D3690, Firstek scientific Model-B403.

The results are shown in Table 1: TABLE 1 Example 1 Example 2 Example 3Conventional Appearance thick and thick and thick and thin and weakstrong strong strong Folding lines none Mild mild deep Peeling 2.5 kg/cmup 2.5 kg/cm up 2.5 kg/cm up 1.6 kg/cm Abrasion 3500+ 2920+ 3500+ 1750+Hydrolysis 1 week up 1 week up 1 week up 2 days resistance

While embodiments of the present invention have been illustrated anddescribed, various modifications and improvements can be made by personsskilled in the art. The embodiments of the present invention aretherefore described in an illustrative but not restrictive sense. It isintended that the present invention is not limited to the particularforms as illustrated, and that all the modifications not departing fromthe spirit and scope of the present invention are within the scope asdefined in the appended claims.

1. A method of producing artificial leather, comprising steps of: (a)providing a substrate; (b) providing a foamed polyurethane dispersion,wherein the polyurethane dispersion has a solid content of about 40% toabout 60% and a void content of about 0.01% to about 80%; (c) coatingthe substrate with a layer of the foamed polyurethane dispersion; and(d) drying the substrate and the layer of the foamed polyurethanedispersion for obtaining the artificial leather.
 2. The method accordingto claim 1, wherein the substrate is pretreated by impregnating orcoating the substrate with a resin solution comprising acrylate resin orpolyurethane resin.
 3. The method according to claim 2, wherein theacrylate resin comprises polymethacrylate, polymethyl methacrylate oracrylic resin.
 4. The method according to claim 2, wherein thepolyurethane resin comprises organic polyurethane resin or polyurethanedispersion resin.
 5. The method according to claim 1, wherein thepolyurethane dispersion has a solid content of 50% to 60%.
 6. The methodaccording to claim 1, wherein the polyurethane dispersion as a solidcontent of about 55%.
 7. The method according to claim 1, wherein thepolyurethane dispersion has a void content of about 2% to about 65%. 8.The method according to claim 1, wherein the foamed polyurethanedispersion further comprises a dye, a thickener, an antioxidant, aflame-resisting agent, an anti-yellowing agent, an antibiotic, or themixture thereof.
 9. The method according to claim 1, wherein the foamedpolyurethane dispersion is formed by mechanical stirring.
 10. The methodaccording to claim 1, wherein the foamed polyurethane dispersion isformed by using a foaming agent.
 11. The method according to claim 10,wherein the foaming agent is selected from the group consisting ofsodium bicarbonate, ammonium carbonate, ammonium bicarbonate, sodiumborohydride, sodium nitrite, polyvinyl chloride, ethylene vinyl acetate,N-nitroso compounds, azo compounds, sulfonhydrazide, and the mixturethereof.
 12. The method according to claim 1, wherein the substrate andthe foamed polyurethane dispersion are dried by heating.
 13. The methodaccording to claim 1, which further comprises: (e) post-treating theartificial leather by the step of adhering a layer, stamping, grinding,printing, or spraying on a surface of the artificial leather. 14-20.(canceled)